Some vehicles may include a hybrid propulsion system which comprises an internal combustion engine and a secondary propulsion source such as an electric motor. These hybrid propulsion systems typically include an energy storage device that may be used to power the secondary propulsion source. In the context of a hybrid electric vehicle (HEV), the energy storage device may include batteries or capacitors that can be discharged to power an electric motor in order to propel the vehicle with or without assistance from the engine. The engine may also be operated to charge the energy storage device by operating a generator.
The inventors herein have recognized several issues with this approach. For example, as a charging operation is initiated, the generator may begin to absorb at least some of the engine output, which may reduce the amount of propulsive effort that is available to the drive wheels. As such, a vehicle operator may perceive a reduction in the responsiveness of the vehicle when the engine is being used to charge the energy storage device. In other examples, initiation of a charging operation may be accompanied by transmission shifting in order to maintain sufficient torque at the drive wheels. However, transmission shifting may also be perceived by the vehicle operator as the energy storage device is cycled in and out of its charging state. Furthermore, transmission shifting may additionally reduce the overall efficiency of the hybrid propulsion system.
To address some of these issues the inventors have provided a method of operating a hybrid propulsion system for a vehicle, including an internal combustion engine, a generator, and an energy storage device. As one example, the method comprises: operating the engine, during a first condition, in a two stroke cycle to produce a first engine output and charging the energy storage device by absorbing a first amount of the first engine output at the generator; and during a second condition, operating the engine in a four stroke cycle to produce a second engine output. In some examples, the engine may not be used to charge the energy storage device while operating in the four stroke cycle. The engine may also be transitioned between operation in the two stroke cycle and operation in the four stroke cycle while maintaining the transmission in the same gear ratio.
As another example, a hybrid propulsion system for a vehicle is provided. The hybrid propulsion system comprises: an internal combustion engine configured to produce an engine output, said engine operatively coupled with one or more drive wheels of the vehicle via a transmission; a motor configured to produce a motor output, said motor operatively coupled with said one or more drive wheels of the vehicle; an energy storage device configured to selectively supply energy to the motor; a control system configured to: increase a level of propulsive effort provided to said one or more drive wheels during a lower state of charge of the energy storage device by reducing a number of strokes performed by the engine per combustion cycle to increase the engine output produced by the engine; and increase a level of propulsive effort provided to said one or more drive wheels during a higher state of charge of the energy storage device by increasing an amount of energy supplied to the motor from the energy storage device to increase the motor output produced by the motor.
In this way, the engine may be operated in a two stroke cycle while performing a charging operation to take advantage of the increased power density of the engine as compared to the four stroke cycle. The increased power density of the engine can enable the charging operation to be performed while maintaining the previously selected gear ratio at the transmission, thereby reducing the number of shifts performed by the transmission. Still other examples and advantages will be described in greater detail herein.